Mobile computing devices have been widely adopted in recent years. Many functions previously performed primarily by personal computers, such as web browsing, streaming, and uploading/downloading of media are now commonly performed on mobile devices. Consumers continue to demand smaller, lighter devices with increased computing power and faster data rates to accomplish these tasks.
Many mobile devices include multiple antennas to provide data rates that satisfy consumers' ever-increasing requirements for upload and download speeds. Integrating multiple antennas into a small form factor device such as a mobile phone or tablet creates the possibility of electromagnetic coupling between antennas. Such electromagnetic coupling has many disadvantages. For example, system efficiency is reduced because signal energy radiated from one antenna is received by another device antenna instead of being radiated toward an intended target. Coupling between antennas becomes even more problematic when the antennas operate at the same or similar frequency bands.
Decoupling networks have been used to decouple antennas from each other. Typically, because a transmitted signal is known, an out-of-phase version of the transmitted signal can be fed to other antennas to which the transmitted signal is electromagnetically coupled. This creates destructive interference that decouples the antennas.
Conventional decoupling networks, however, suffer from several substantial drawbacks. For example, conventional decoupling networks operate at a single frequency. This prevents devices with antennas operating at multiple frequency bands from being simultaneously decoupled for all of the multiple frequency bands. Additionally, the out-of-phase signal used for decoupling is conventionally created using lengths of transmission line that provide the required decoupling conditions. The length of transmission line necessary to create the decoupling conditions is frequency dependent, which not only limits the decoupling network to one frequency of operation but creates space concerns for lower frequencies in smaller form factor designs.